Release of weak and strong copper-complexing agents by algae1

By means of potentiometric titrations, the copper-complcxing properties of organic compounds released by 21 algal species are characterized. Most eucaryotic species relcascd measurablc amounts (lo-” M) of copper-complexing agents that could be modeled as weak organic acids with log{*&.,,,} = 0.5. Of seven blue-green algal species studied, four produced about 5 x lo+ M concentrations of strong copper-complexing agents with conditional stability constants in the range lOa to 10 12; two produced complcxing agents similar to those produced by the eucaryotes. In time-course experiments with three chlorophytes and two cyanophytes, the release of measurable concentrations of copper-complcxing agents occurred principally during stationary phase. Trace metals can control algal growth by acting either as toxicants (e.g. copper), or as limiting micronutrients (e.g. iron). Toxicity of and nutrient limitation by trace metals depend on their chemical speciation in the medium (Sunda and Guillard 1976; Anderson and Morel 1978; Anderson et al. 1978). In turn, the speciation of metals in natural waters and in culture media is controlled by chemical processes such as precipitation, adsorption, and inorganic and organic complexation (Stumm and Morgan 1970; Morel et al. 1979u). By releasing metabolites that complex metals, algae could, in principle, modify metal speciation in the medium and effectively control metal availability or toxicity in their external milieu. To have a significant effect on the trace metal chemistry of natural waters, an organic ligand must have relatively high affinity for metals; greater, for example, than that of simple amino acids and organic acids (Stumm and Morgan 1970; Stumm and Brauncr 1975; Morel et al. 1974; Jackson and Morgan 1978). Hydroxamic acids may be the only examples of strong metal-complexing agents that are known to be released by algae and to be present in natural waters (Anderegg et 1 This work was supported in part by the International Copper Research Association Project 252, by the Office of Sea Grant (NOAA), U.S. Department of Commerce, grant 04-7158-44079, and by NSF grant OCE-7709000. al. 1963; Neilands 1967, 1973, 1974; Simpson and Neilands 1976; Murphy et al. 1976). Simpson and Neilands (1976) identified a polyhydroxamate siderochrome, called schizokincn, produced by Anabaena sp. and Murphy et al. (1976) found bound hydroxamates in cultures of Anabaena flos-aquae and in field samples from blooms of Anabaena sp. The role of siderochromes in microbial iron transport has been established from studies of iron mutants of Salmonelk typhimurium (Luckey et al. 1972) and Bacillus megaterium (Davis and Byers 1971) and presumably a similar transport mechanism exists in blue-green algae. Fogg and Westlake (1955) demonstrated complexation and detoxification of copper by extracellular polypeptides from cultures of Anabaena cylindrica. They found that the pH titration of a copper sulfate-extracellular polypeptide solution was depressed relative to the sum of separate pH titrations of copper and polypeptide solutions and that the addition of extracellular polypeptide decreased the toxicity of copper to cultures of A. cylindrica. Since then, researchers have elucidated the importance of organic complexation in determining copper toxicity to algae. Using synthetic chelators whose copper-complexing properties are known, several researchers have shown that the toxicity of copper to several marine algae is a unique function of the cupric ion activity (Sunda and Guillard 1976; Anderson and Morel 1978).